Direct Observation of Fe-Ge Ordering in Fe_{5− x}GeTe₂ Crystals and Resultant Helimagnetism

Microscopic structures and magnetic properties are investigated for Fe_{5− x}GeTe₂ single crystal, recently discovered as a promising van der Waals (vdW) ferromagnet. An Fe atom (Fe(1)) located in the outermost Fe₅Ge sublayer has two possible split-sites which are either above or below the Ge atom. Scanning tunneling microscopy shows √3 × √3 superstructures which are attributed to the ordering of Fe(1) layer. The √3 × √3 superstructures have two different phases due to the symmetry of Fe(1) ordering. Intriguingly, the observed √3 × √3 ordering breaks the inversion symmetry of crystal, resulting in substantial antisymmetric exchange interaction. The temperature dependence of magnetization reveals a sharp magnetic anomaly suggesting helical magnetism of the Fe_{5− x}GeTe₂ due to its non-centrosymmetricity. Analytical study also supports that the observed ordering can give rise to the helimagnetism. The work will provide essential information to understand the complex magnetic properties and the origin of the new vdW ferromagnet, Fe_{5− x}GeTe₂ for future topology-based spin devices.